Age and signal strength-related changes in vessel density in the choroid and the retina: an OCT angiography study of the macula and optic disc

PURPOSE To investigate the effects of ageing and signal strength index (SSI) on vessel density (VD) measurements of the radial peripapillary capillaries (RPC) in the optic disc, the retinal superficial (SCP) and deep (DCP) capillary plexi and the choriocapillaris (CC) of the macular area using optical coherence tomography angiography (OCT-A) imaging. METHODS This cross-sectional study included 138 eyes from 77 healthy participants. High-definition (HD) disc Angio scan (4.5X4.5 mm) and HD Angio retina scan (6X6 mm) were acquired. Whole image (WI) VDs, inside disc (ID) VD and global and hemifield peripapillary RPC VDs were evaluated from the Angio Disc scans. Global and hemifield VD for the SCP, DCP and CC slab in the macula area were also recorded across different age groups. RESULTS Mean age was 48.2 ± 15.0 years (range 25-82 years). Signal strength index (SSI) was 71.6 ± 7.6 in the macula and 74.6 ± 9.2 in the disc OCT-A image. VD for both SCP and DCP decreased by 0.084% and 0.188%/year (both p < 0.001). WI VD and ID VD also decreased by 0.037%/year (p = 0.005) and 0.092%/year (p < 0.001). Peripapillary RPC VD did not change with age or SSI. The effect of SSI was significant in SCP, DCP, WI RPC, ID RPC and CC VD. In the generalized estimating equation model testing for both SSI and age, SSI remained significant against SCP, DCP and ID RPC. CONCLUSION With ageing, a decrease in VD in the SCP, DCP, WI and ID RPC, as well as in SSI, is seen. SSI and age are associated with VD changes, but only SSI is the significant variable that explains the variation in the retina microvasculature in the macula and the optic disc in multivariate analysis. In the CC, a larger SSI was associated with a lower VD. These findings underscore the importance of controlling for SSI in future studies using OCT-A.